My group has about a six hour trip to Otakon, give or take. We ALWAYS stop around halway in Hagerstown to get a full meal, and stop about once an hour to use a bathroom and get a drink. We'll stop pretty much anywhere, gas stations and fast food places are everywhere, so once you start feeling like you need to move around, just start looking for those signs before interstate exits that tell you what food and gas places are coming up!
Also, if you do a pit stop, make sure you try to find exits where the place to get back on is fairly obvious - nothing more annoying than getting off the interstate to pee and then having to spend an hour navigating some unknown city to get back on the road!
We'll generally just listen to music the whole way by filling my boyfriend's MP3 player with music we can all agree on (or popping my phone onto Pandora and hooking that up), and the occassional conversation.
Aside from just enjoying the drive, I'll usually sit and read on my Kindle, or play something on the DS. I feel silly dragging lots of stuff in the car when I know very well (after years of traveling for band/choir) that I will use NONE of it and it's just taking up space!